1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with apparatus whereby a loudness signal in combination with a formant control signal produces tones with time variant spectral characteristics which are selected in response to the loudness signal.
2. Description of the Prior Art
An elusive goal in the design of electronic musical instruments is the ability to realistically imitate the sounds of conventional acoustic type musical instruments. The best results have been obtained for electronic musical instruments which imitate wind-blow pipe organs and harpsichords. The principal reason for obtaining good imitative results for these instruments is that they are essentially mechanical tone generators. The tone generation is automatic and the musician only actuates on-off switches. With the notable exception of these two instruments, the tone character of almost all other musical instruments is a function of certain skills possessed by the musician.
It has long been recognized that with the notable exception of conventional organ tones, almost all tones from musical instruments exhibit tone spectra which are time variant. The recognition of the characteristic time variant spectra has motivated the development of electronic musical generating systems such as those known by the generic names of "sliding formant" and "FM-synthesizer." Sliding formant tone generators constitute a class of generators which are also called subtractive synthesis. In subtractive synthesis, the fundamental tone source generates more than the desired tone spectral components and the undesired spectral components are attenuated, or filtered out, by means of some variety of frequency filter. The FM-synthesizer is of the additive variety in that FM (frequency modulation) is used to add components to a source signal which often consists of a simple single frequency sinusoid time function.
The imitation of acoustic orchestral musical instruments using synthesis techniques such as the sliding formant or FM-synthesizer has been of the trial and error variety. One adjusts a multiplicity of tone controls and ADSR envelope controls until an output tone is produced which is judged to be "near to" or imitative of some particular preselected musical instrument. These techniques are distinctly different from the more academic procedure in which one first analyzes the tones from a selected musical instrument, then constructs an analytical model of the tone generator, and finally uses the experimentally obtained parameters in the analytical model to synthesize tones that closely imitate the original.
The academic approach of analysis, modeling, and synthesis for musical instrument sounds is obviously not an easily implemented technique judging from the current lack of success except in a few relatively isolated instances. Part of the problem rests in the inability to adequately model many of the subtleties in tone structure imparted by the musician in an effective performance on his instrument. The musician commonly uses a technique such that the tonal structure for a given musical note varies with the loudness of the tone. Repeated notes are played with different loudness and tonal structure and this subtle difference removes the mechanical-like repeated tones produced by most electronic musical tone generators. Even very expert players are unable to repeat a given tone with precisely an identical tone spectra. In general, as the tone level becomes louder, the tone spectra increases in the number and strength of the higher harmonics. Very soft tones tend to approach tones having only a few harmonics.
To compound the problem of imitating musical instruments is that even a sustained tone does not have a constant tone spectra. For almost all instruments it is well-known that the spectrum will change with time and with the tone's instantaneous amplitude envelope. Such time variant spectral variations are introduced in electronic musical instruments using synthesis techniques such as the sliding formant and FM-synthesizer.
In the copending patent application Ser. No. 139,908 filed Apr. 14, 1980 entitled "A Polyphonic Tone Synthesizer With Loudness Spectral Variation" a system is disclosed whereby a loudness control system is used to vary the spectral content of generated musical tones. The referenced patent application and the present invention are assigned to the same assignee. The invention disclosed in the application describes means for causing a tone generator of the type described in U.S. Pat. No. 4,085,644, entitled "Polyphonic Tone Synthesizer," to produce musical tones having spectral components which can be made to be time variant in response to a control signal such as the note's ADSR envelope function or the setting of a loudness control.
In a Polyphonic Tone Synthesizer of the type described in the referenced patent, a computation cycle and a data transfer cycle are repetitively and independently implemented to provide data which are converted into musical waveshapes. During the computation cycle a master data set is generated having a spectral content which is variable in response to an input spectral control signal. This is accomplished by utilizing a table of stored sinusoid values to address data values from a transform memory which contains a preselected set of data points. The sinusoid function values are scaled in magnitude in response to the input spectral control signal thereby causing a variable subset of the transform memory contents to be read out and stored in a main register. These computations are executed at a fast rate which may be nonsynchronous with any musical frequency.
Following a computation cycle, a transfer cycle is initiated during which the master data set is first adjusted to have a zero average value and is then transferred to selected members of a multiplicity of note registers. The data residing in the note registers are read out under control of note clocks having frequencies corresponding to the assigned corresponding keyboard switches. This data is read out sequentially and repetitively and converted to analog musical waveshapes. Tone generation continues uninterrupted during the computation and the transfer cycles.
An objective of the present invention is to provide a means of generating the data stored in the transform memory using preselected sets of harmonic coefficients during a subsequence of the computation cycle.
It is well-known in the musical tone generation art that time variant spectral variations of tones produce desirable tonal effects. These tonal effects range from imitations of musical instruments to novel tones having no imitative parents. The usual method of producing such spectral variations is to use the sliding formant techniques. This technique is usually implemented by using a frequency-domain filter with controllable cut-off frequencies to process waveshapes generated by analog musical instruments.
A different type of sliding formant tone generation is described in U.S. Pat. No. 3,908,504 entitled "Harmonic Modulation And Loudness Scaling In A Computer Organ" and in the previously referenced U.S. Pat. No. 4,085,644. In these systems the time variation of spectral content is obtained by varying the magnitude of harmonic coefficients used to compute waveshape amplitude points by means of a Fourier-type algorithm. The tonal effects obtained by "sliding" the magnitude of selected harmonic coefficients are very similar to tonal effects obtained by "sliding" the cut-off frequencies of a frequency domain filter.
An application of nonlinear system transformations in the generation of musical sounds is contained in the technical article: Beauchamp, J., "Brass Tone Synthesis by Spectrum Evolution Matching with Nonlinear Functions." Computer Music Journal, Vol. 3, No. 2, pp 35-42 (1979).